There is no admission that the background art disclosed in this section legally constitutes prior art.
It is well known that solar cells or photovoltaic cells (PV cells) can be used to convert solar energy into current and voltage, or electric energy. Typical photovoltaic cells include a substrate and two ohmic contacts (i.e., electrode layers) for passing current to an external electrical circuit. The cell also includes an active semiconductor junction, usually comprised of two or three semiconductor layers in series. The two-layer type of semiconductor cell consists of an n-type layer and a p-type layer. The three-layer type includes an intrinsic (i-type) layer positioned between the n-type layer and the p-type layer for absorption of light radiation. The photovoltaic cells operate by having readily excitable electrons that can be energized by solar energy to higher energy levels, thereby creating positively charged holes and negatively charged electrons at the interface of various semiconductor layers. The creation of these positive and negative charge carriers creates a net voltage across the two electrode layers in the photovoltaic cell, and establishes a current of electricity.
In order to improve the performance of photovoltaic cells, it is advantageous to decrease the thickness of the various protective and active layers to reduce materials usage and weight, improve mechanical flexibility, and improve light transmissibility into and through the structure. Thin-film and ultra-thin-film photovoltaic cells utilize thin-film semiconductor materials to form the various active layers of the cells. These thin film semiconductor materials offer several distinct advantages over thicker crystalline materials, insofar as they can be easily and economically fabricated into a variety of devices by mass production processes.
As the thickness of the various functional layers decreases, the impact of defects within the layer structures and at the layer junction interfaces becomes more pronounced. One such defect is the presence of current-shunting, short circuit defects. These defects seriously impair the performance of the photovoltaic devices fabricated from thin film semiconductor materials and also detrimentally affect production yield. These process-related defects are thought to either be present in the morphology of the substrate electrode, or develop during the deposition or subsequent processing of the semiconductor layers.
Shunt defects are present in photovoltaic devices when one or more low resistance current paths develop through the semiconductor body of the device, allowing current to pass unimpeded between the electrodes thereof. Under operating conditions, a photovoltaic device in which a shunt defect has developed, exhibits either (1) a low power output, since electrical current collected at the electrodes flows through the defect region (the path of least resistance) in preference to an external load, or (2) complete failure where sufficient current is shunted through the defect region to “short out” the device.
In certain instances, barrier layers may provide passivation of shunt defects. U.S. Pat. No. 4,251,286 to Barnett shows PV cells having n- and p-layers made from copper sulfide and cadmium sulfide forming a heterojunction interface. In one embodiment, the PV cells may include a blocking layer (16) of zinc sulfide formed between the collector p-layer (12) and the back contact (14). In another embodiment, the blocking layer may be a localized cadmium sulfate layer (17) disposed on the exposed portions of the cadmium sulfide active layer.
U.S. Pat. No. 4,598,306 to Nath et al. shows barrier layers formed from oxides, nitrides and carbides of indium, tin, cadmium, zinc, antimony, silicon, and chromium. These barrier layers are transparent and preferably applied between a transparent electrode and the semiconductor body. Such barrier layers were applied to amorphous silicon p-i-n-PV cells having a stainless steel back contact.
U.S. Pat. No. 7,098,058 to Karpov et al. discloses a method of applying a bonding material onto a semiconductor layer having electrical non-uniformities. The formed semiconductor layers of a photovoltaic cell are immersed into a solution of the bonding material and an electrolyte. The semiconductor layers are energized by way of light energy causing an electrical potential within the semiconductor layers. The bonding material is carried to the semiconductor surface by an electrochemical reaction that causes a redistribution of the positive and negative ions of an electrolyte solution. The bonding material becomes selectively applied to areas of aberrant electric potential by way of the energized semiconductor layers.
Thus, it would be desirable to provide a buffer or barrier layer that is compatible with CdS/CdTe flexible, thin film PV cells that is effective for CdTe layers of about 0.5 microns or less and convenient to apply.